The present invention relates to an atmospheric pressure ionization mass spectrometer and more particularly to an atmospheric pressure ionization mass spectrometer which is suitable for analyzing impurities contained in silicon group gases which generate solid deposits by ionization by discharge and have a property to pollute the ion source thereby.
As refinement of semiconductor devices has advanced recently, even trace impurities contained in gases may affect device performance. Therefore, it is necessary to supply highly purified gas in the production process and a highly purified gas analyzer is essential to manufacture and management of this gas. An atmospheric pressure ionization mass spectrometer (APIMS) can analyze trace impurities (on a level between ppb and ppt) in gases such as nitrogen and argon, so that it is an essential measuring instrument in the semiconductor device manufacturing process.
A conventional APIMS is, for example, as shown in FIG. 15. This apparatus is described in U.S. Pat. No. 4,023,398. In an ion source 151, sample gas 2 at almost 1 atmosphere is ionized by discharge or radiation. Formed sample gas ions cause an ion-molecule reaction and trace components contained in the sample gas are ionized highly efficiently. Formed trace component ions are transmitted by the electric field, pass through a gas curtain chamber 152, pass through an aperture furthermore, enter an analysis region 24, and then are mass-separated and detected. To prevent the sample gas 2 from entering the analysis region 24, curtain gas 153 is controlled so as to flow into the ion source 151 from the gas curtain chamber 152.
In FIG. 15, a reference numeral 154 indicates liquid helium, 155 a cryopump, and 156 flow of curtain gas.
Another apparatus is as shown in FIG. 16. This apparatus is described in Japanese Patent Application Laid-Open No. 60-241634. The apparatus consists of an ion formation region 5, a sample inlet 161, and a mass analysis region 24 which has an aperture 19 for introducing ions and is exhausted by a vacuum pump. The ion formation region 5 discharges ion formation gas such as nitrogen gas and forms ions. Formed ions are gushed into the atmosphere together with ion formation gas. On the other hand, sample gas 2 is gushed from the sample inlet 161. Immediately before the aperture 19, ions are mixed with sample gas 2, collide with molecules of target trace component contained in the sample gas, cause an ion-molecule reaction, and ionize the target component. Formed target component ions are sucked in the mass analysis region 24 under high vacuum via the aperture 19 and mass-separated by a mass analysis means and detected.
In FIG. 16, a reference numeral 1 indicates primary ion formation gas, 162 a mixing region of sample gas and primary ions, and 163 a mixture of primary ions and primary ion formation gas.
Still another apparatus is as shown in FIG. 17. This apparatus is described in Japanese Patent Application Laid-Open No. 60-241634. The apparatus consists of an ion formation region 5, a differential pumping region 171, a sample inlet 161, and a mass analysis region 172 which has an aperture 19 for introducing ions. The ion formation region 5 discharges ion formation gas such as nitrogen gas and forms ions. Ions formed by the ion formation region are sucked in the differential pumping region 171 together with gas, mixed with sample gas there, and cause an ion-molecule reaction. The target trace component contained in the sample gas is ionized. Target component ions formed by the differential pumping region 171 pass through the aperture 19, and enter the mass analysis region 172, and are mass-separated and detected.
Furthermore, in U.S. patent application Ser. No. 08/016,534 which was filed on Feb. 11, 1993 and is now placed in condition for allowance, a further apparatus shown in FIG. 18 is described. This apparatus consists of an ion source 285 and a drift tube 296 and the ion source 285 is separated into three chambers. Ions of purified gas 290 which are formed in the first chamber farthest from the drift tube 296 are transmitted by the electric field and introduced into the third chamber closest to the drift tube 296. In the third chamber, ions of purified gas 290 cause an ion-molecule reaction to sample gas 281 and ions of the main component of sample gas 281 are formed. Ions of the main component of this sample gas 281 are detected by the drift tube 296. The amount of ions of the main component of sample gas 281 varies with the concentration of impurities contained in the sample gas, so that by measuring this amount, the concentration of impurities contained in the sample gas can be measured.
In FIG. 18, a reference numeral 281 indicates sample gas, 282 and 287 pressure regulators, 283, 288, and 328 flow controllers, 284 a needle electrode, 285 an ion source, 286, 291, 311, and 327 exhaust gases, 289 and 329 purifiers, 290 purified gas, 292 and 292' ion extraction electrodes, 293 a shutter, 294 an electrode, 295 a detector, 296 a drift tube, 305, 306, 310, and 326 outlets, 307, 308, 316, and 325 insulators, and 324 an ion source container.
In semiconductor device manufacturing processes, many types of gases are generally used. However, since silicon group gases such as monosilane and disilane are used as materials of semiconductor layers, particularly high purification is important. Impurities such as water and oxygen cause formation of dust and formation of native oxide films in the reactor which are factors of process defects. It is requested to reduce these impurities below a ppb level. For that purpose, a super high sensitive gas analysis art for evaluating those impurities on a level between ppt and ppb is essential. Regarding so-called stable gases such as nitrogen, oxygen, and argon, the conventional APIMS can analyze impurities on a ppt level. However, regarding silicon group gases, it cannot analyze impurities on a ppt level.